Undervoltage protection device

ABSTRACT

An undervoltage protection device for a voltage source and a load wherein a first and second switch are connected in a current path established between the voltage source and the load, with each of the switches being adapted to provide an open circuit in the current path. A first circuit is provided for closing the second switch in response to the first switch being closed, the circuit being operable, unless disabled, to open the second switch a predetermined time interval after the first switch closes. An additional circuit disables the first circuit in response to a predetermined minimum output from the voltage source.

O i I Umted States Patent [151 3,648,145 Meyer et a1. 5] Mar. 7, 19 72 [54] UNDERVOLTAGE PROTECTIO 3,447,060 5/ 1969 Tedd .Q ..320/40 X DEVICE 3,395,288 7/1968 Von Brimer.. .....320/40 X 3,47 ,296 19 R [72] inventors: Bennett A. Meyer, Port Chester, N.Y.; 3 043 y x v Kenyon E B. McGuire Demarest Ni [73] Assignee: The Singer Company, New York, NY. Primary Examiner-William M. Shoop, Jr. [22] Filed: May 8, 1970' Attorney-S. A. Giarratana and Thomas W. Kennedy [211 Appl. No.: 35,701 [57] ABSTRACT An undervoltage protection device for a voltage source and a [52] US. Cl. ..320/l3, 317/123, 317/ 148.5 load wherein a first and second switch are connected in a cur- 21 rent path established between the voltage source and the load, Fleldolsfllfll 3 with each of the switches being adapted to provide an open 3l7/DIG- 148-5; 320/13 29, 40 circuit in the current path. A first circuit is provided for closing the second switch in response to the first switch being [56] Rdem cued closed, the circuit being operable, unless disabled, to open the UNITED STATES PATENTS second switch a predetermined time interval after the swltch closes. An additional circuit disables the first circuit m 3 3 3/1967 Mol'gan 321/45 x response to a predetermined minimum output from the voltl Savage X age source 3,243,658 3/1966 Blackburn... ....317;317/123;D1G. 5 3,321,754 5/1967 Grimm et a1. ..317;317/ 123;D1G. 5 1 Claims, 1 Drawing Figure Patented March 7, 1972 3,648,145

INVENTORS BENNETT A. MEYER & KENYON E.B. MCGUIRE ATTORNEY UNDERVOLTAGE PROTECTION DEVICE BACKGROUND OF THE INVENTION The invention herein described was'made in the course of or under a. contractor subcontract thereunder,with the Department of the Air Force.

This inventionrelates to an undervoltage, protection-device and, more particularly, to a devicezfor connecting between a voltage source and a loadv to preventthe voltagesource from being used when. its terminal voltage falls below a certain value.

When using a voltage source, such asa nickel-cadmium battery, to energize portable equipment it is necessary to monitor the battery voltage and prevent the battery from being. used when its terminal voltage falls below a certain value. if the battery. is used when its voltage has fallenbelow the criticalvalue, permanent and irreversible. damage will result to the battery due to cell reversal. Moreover,the-equipment which is energized by the battery'shouldnot function when theterminal voltage of the battery fallsbelow a certain value in order-to prevent possible damage to the-equipment.

Several proposalshave been made for preventing a battery from being usedwhen its terminal voltagefalls below a certain value. For example, avoltage-sensing circuit has'been devised which. is similar in design to a voltage regulator and requires a relatively high numberof components. Furthermore, this type of. circuit draws current from the battery-even. after the load circuit has been turned ofi". Otherxproposals. require. a specifically manufactured tapped battery and, in addition, .do not provide a firm turnoff :point. Thus the circuit can operate in a partially on mode, resulting in equipmentmalfunctions.

.Still otherproposals utilize a voltmeter as part of thebattery-powered equipment, which requires the operator to continually monitor the battery voltage and stop the operations of theenergized equipmentwhen-the battery voltage falls below thecritical value. In addition to being expensive, the requirement of the constant presence and scrutiny ofan operator may be impossible to fulfill under certain operational surroundings.

SUMMARY OF THE INVENTION It is therefore an object of thepresent invention to provide a I protection devicefor connection between a voltage source and a load which disconnects the voltage source from the load when the terminal voltage from theivoltagesource falls below a critical value.

-lt isa-further object of the present invention to provide a protection device of theabove type in-which astandard-battery and a'minimum' of component parts areutilized.

. device of the presentinvention.comprisesmeansrfor establishing a current path-between a voltage source and itsload, first,

and second switching means connected insaidcurrentpath,

.open. the switch :20, a predetermined time interval after the each. adapted to move .toan open position to providean open circuit in said currentpath, first circuit means responsive to said first switchingmeans being closedfor closingsaid'second switching means, said first circuitmeans being .operable,-un-, lessdisabled, to'open said second switching means closed a predetermined time intervaLafter said first switchingmeans closes,-and a second circuit-means responsive'to-a predetermined voltage output from'said voltage source to disable said .contemplated for carrying out the objects of thev invention and are not to be construed as restrictions or limitations on its scope. The drawing is a circuit diagram of the protection device of the present invention.

DESCRIFI'IONOF BREFERRED EMBODIMENTS Referring, to the drawing, a battery 10 is shown which is connected to a load 12 by means of a pair of conductors l4. and 16. The conductor 14 is made up of three separate conductors,;l4 a, 14b. and 14c, with a switch 18 extending'between the conductors 1.4a and l4b,.and a relay switch 20 extending between the conductors 14b and 140. The switch 18 connects the conductors 14a and 14b in its on" or closed position,

while; it provides an open. circuit in its ofi' or open position. Similarly, the switch 20 connects conductors 14b and Me in its closed position andprovides an open circuit in its open position. The-switch 18 is manually operated between its'two positions, .while theswitch 20. is normally open and is operated in amanner to-be described later.

A pair of resistors 22 and 124 are connected between the conductors 14b and 1 6,, with the resistor22 having a very high value, suehas l. megohm, and the resistor 24 having a relative-- ly low value. .A, capacitor 26 connected between the conduc- .tor 1411; and a resistor 30, which, in tum, is connected to the base of a NPN-switching-transistor 32. The resistor 30 is of anf intermediate value ,with respect to the values of the resistors 22 and;24.

The emitter of the transistor 32 is connected to the conductor 16, while the collector is connected to a relay coil 36 .which, in turn, is connected to the conductor 14b and is adapted to operate the relay switch 20 in a conventional manner.

A;di0de38 andazener diode40 are connected in series, via a conductorx42, to a connector, 44 extending between the conjunction between the resistors 22 and 24, and the junction between the capacitor26 and the resistor 30. The zener diode operates to. conduct. only when a predetermined minimum voltage is present across the conductors 14c and 16.

In operation, .and assuming that the'switch 18 is in its open position,.and the switch;.20 is in its normal open position, the switch '18 is manually-rnoved to its closed position as shown, wherebyasubstantial portion ofthe current flowing from the battery: l0.will passthrough the. switch into a circuit including :the. capacitor 26 and the resistor,30, thus applying a potential to the base of the. transistor 32 to turn it on. This continues as long as it takes the capacitor 26 to completely charge, and, as

minal voltage from therbattery to beapplied to the diodes 38 ,and40 andtotheloadlZ.

. when the capaciton26 becomes completely charged, current ceases .to flow therethrough, and due .to the presence of the veryzhigh resistor-22am the relatively, low resistor 24, current flowto the base .oflthe transistor 32 is cut olT/l'hus the above circuit, including the capacitor. 26,-normally operates to switch. l8 closes. However,assuming that the voltage from the battery -is.-.above a predetermined I critical value, the zener ,diodeadmwill conduct current from the conductor 14, through the conductor-.42, and to;the conductor-44 whereby current of a sufficientxmagnitude to maintain the transistor 32 turned on .will be applied toits base via the resistor 30, and the coil 36 AWIII remain energized to maintain the switch 20 closed. Thus first circuit means from'opening said second switchingmcans. helmet ckcuitincluding the uncldiode tru to di BRIEF DESCRIPTION OF THE DRAWING "Reference is -now made to the.accompanying.drawingfor a better understanding of the nature andobjects of the present .ablethe circuit, including thecapacitor 26, from opening the switch'20.

As long as thevoltage from .the battery equals or exceeds the critical .valueforthe zener diode, the circuit will operateas invention. The drawing illustrates the best mode presently described immediately above, andthe conductor 42 will apply the current necessary to maintain the transistor 32 turned on, and the switch 20 closed. If, at any time, the voltage of the battery falls below the critical value, the zener diode 40 will not conduct and therefore there will be no current applied to the base of the transistor 32 after the capacitor 26 has become completely charged. As a result, the coil 36 will be deenergized, the switch 20 will be released to move to its normally open position, and the load 12 will be disconnected from the battery 10. The circuit can then be reactuated by operating the switch 18 to its open position to discharge the capacitor through the resistors 22 and 24, and then to its closed position whereby the above sequence will be repeated.

The diode 38 prevents the capacitor 26 from charging through an unwanted circuit path through the zener diode 40 and the load 12. The elimination of this unwanted circuit path increases the stability of the operating time of the relay switch 20 and enables the circuit to perform with a wide range of load currents.

Of course, if at any time it is desired to disconnect the load from the battery, the switch 18 is simply moved to its open position, which causes the capacitor 26 to discharge as described above, whereby the circuit will be ready for immediate actuation.

The device of the present invention has several advantages. It draws no current from the battery when the battery is disconnected from the load except through the very large resistor 22, and consumes a small amount of current while it operates to connect the battery to the load. Also, it will not chatter when the battery voltage is at the critical threshold, and will not automatically connect the battery to the load as a result of the battery voltage spontaneously rising because of a no load condition, after the device has operated to disconnect the battery from the load.

Further, the device of the present invention can be recycled by operating the power switch, and its operation is automatic with no operators attention required. Still further, no battery taps are required and a small number of components are utilized.

Also, the voltage threshold stability of the device of the present invention is limited solely by the zener diode, and it removes the load with a positive switching action immediately after the battery voltage falls below the critical value.

It is understood that several variations in the above may be made without departing from the scope of the invention. For example, in order to increase the stability of operation over a range of temperatures, a number of zener diodes similar to zener diode 40 can be operated in series. These individual diodes should be selected so that the resultant zener voltage is the value desired and the thermal coefficient is 0. Other variations of the specific construction and arrangement of the device disclosed above can be made by those skilled in the art without departing from the invention as defined in the appended claims.

We claim:

I. A protection device for a voltage source and a load, comprising means for establishing a current path between said voltage source and said load, first and second switching means connected in said current path, each adapted to move to an open position to provide an open circuit in said current path, first circuit means responsive to said first switching means being closed for closing said second switching means, said first circuit means being operable, unless disabled, to open said second switching means a predetermined time interval afler said first switching means closes, and second circuit means responsive to a predetermined voltage output from said voltage source to disable said first circuit means from opening said second switching means, wherein said second switching means is in the form of a relay switch, wherein said first circuit means comprises a coil adapted to energize said relay switch to close same and a switching transistor responsive to said first switching means being moved to a closed position for connecting said coil in said current path, wherein said switching transistor is connected in said current ath by means of a capacitor In a manner so that said transls or is turned on only during the vcharging of said capacitor in response to said first switching means being closed, wherein said second circuit means comprises a zener diode connected in said current path between said second switching means and the base of said switching transistor, and wherein said second circuit means further comprises a diode connected in series with said zener diode, said diode connected so as to prevent said capacitor from charging through said load. 

1. A protection device for a voltage source and a load, comprising means for establishing a current path between said voltage source and said load, first and second switching means connected in said current path, each adapted to move to an open position to provide an open circuit in said current path, first circuit means responsive to said first switching means being closed for closing said second switching means, said first circuit means being operable, unless disabled, to open said second switching means a predetermined time interval after said first switching means closes, and second circuit means responsive to a predetermined voltage output from said voltage source to disable said first circuit means from opening said second switching means, wherein said second switching means is in the form of a relay switch, wherein said first circuit means comprises a coil adapted to energize said relay switch to close same and a switching transistor responsive to said first switching means being moved to a closed position for connecting said coil in said current path, wherein said switching transistor is connected in said current path by means of a capacitor in a manner so that said transistor is turned on only during the charging of said capacitor in response to said first switching means being closed, wherein said second circuit means comprises a zener diode connected in said current path between said second switching means and the base of said switching transistor, and wherein said second circuit means further comprises a diode connected in series with said zener diode, said diode connected so as to prevent said capacitor from charging through said load. 